The present invention relates to a process for producing single crystal ingots having a reduced amount of crystal defects. More particularly, the present invention relates to a process for producing a silicon melt for growing single crystal silicon ingots wherein the silicon melt contains a very low amount of gases insoluble in silicon.
In the production of single silicon crystals grown by the conventional Czochralski method, polycrystalline silicon in the form of granular polysilicon, chunk polysilicon, or a mixture of chunk and granular polysilicon is first introduced into a quartz crucible in a crystal pulling apparatus. Chunk polysilicon is a polycrystalline silicon mass which is generally irregular in shape, with sharp, jagged edges as a result of the fact that it is prepared by breaking rods of polycrystalline silicon into smaller pieces; chunk polysilicon typically ranges from about 2 centimeters to about 10 centimeters in length and from about 4 centimeters to about 6 centimeters in width. Granular polysilicon is a polycrystalline silicon mass that is generally smaller, more uniform and smoother than chunk polysilicon as a result of the fact that it is typically prepared by chemical vapor deposition of silicon onto a silicon granule in a fluidized bed reactor; granular polysilicon typically ranges from about 1-5 millimeters in diameter and generally has a packing density which is about 20% higher than chunk polysilicon.
After the loading of the polysilicon, the apparatus is sealed and evacuated using a vacuum pump apparatus to a pressure of about 100 millitorr to remove the ambient air surrounding the crucible. It is generally desirable to evacuate the crystal pulling apparatus prior to heating and melting the polycrystalline silicon to remove moisture and oxygen from within the crystal pulling apparatus to prevent reaction with molten polysilicon. Reactions between moisture and molten polysilicon and/or oxygen and molten polysilicon can produce silicon oxide gas that can condense and accumulate on the furnace tank and hot zone parts in the crystal pulling apparatus. During subsequent crystal growth, this oxide condensate can detach from the furnace tank and/or hot zone parts and fall into the melt resulting in serious defects in the growing crystal making many of the resulting wafers unsuitable for use.
After the ambient air is substantially evacuated from the crystal puller, the vacuum system is typically turned off and the crystal puller is backfilled with a gas which is substantially non-reactive with silicon. Conventionally, argon, which is highly insoluble in silicon, has been utilized. Argon is typically backfilled into the crystal pulling apparatus to achieve a pressure of about 100 Torr. After backfilling, the vacuum system is again engaged and the argon gas is substantially evacuated to achieve a pressure of about 1 Torr. This backfilling and evacuation procedure may be repeated several times to ensure that substantially all of the ambient air is removed from the crystal pulling apparatus.
Following the final evacuation, argon is backfilled in the crystal puller to achieve a pressure of typically from about 10 Torr to about 30 Torr and the pumping and argon flow system are adjusted for pressure and flow conditions for an argon purge utilized during silicon heating and melting and ingot growth. Typical argon purges may be at a pressure of about 15 Torr and a flow of about 50 slm to about 100 slm.
After the evacuation and backfilling procedures are complete, the polysilicon is melted down and equilibrated at a temperature of about 1450xc2x0 C. As the polysilicon is heated and melted, the argon purge gas is continuously introduced over the crucible and silicon to remove unwanted contaminants from the melt area that are produced in and around the melt during the melting of the polysilicon. After the silicon has completely melted and reached a temperature of about 1450xc2x0 C., a seed crystal is dipped into the melt and subsequently extracted while the crucible is rotated to form a single crystal silicon ingot.
During the addition of the chunk and/or granular polysilicon to the quartz crucible, small cavities are created between the polysilicon pieces themselves and between the polysilicon pieces and the crucible bottom and sidewalls. After the ambient air is removed by vacuum from the crystal puller, the backfilled argon gas replaces the ambient air in these cavities. Consequently, when the polysilicon is melted, the argon gas that filled the cavities vacated by the ambient air forms insoluble gas bubbles in the melt. Many of these gas bubbles comprised of argon will remain attached to the crucible bottom and/or sidewalls for many hours during the crystal growing process. Additionally, during the early stages of the melting process when the polycrystalline charge is completely or partially in the solid state, the argon purge gas may become trapped in the above-described cavities in the polysilicon charge.
While the problem of trapped gases occurs with all charge types including chunk silicon, granulated polycrystalline silicon, and mixtures thereof, the problem is particularly acute with charges formed from only granulated polycrystalline silicon; the granular polysilicon with its high packing density tends to increase trapping of gas at the bottom and side walls of the crucible. Because argon is highly insoluble in silicon, trapped argon gas in the melt forms small bubbles in the liquid silicon during melting. Many of the insoluble gas bubbles contained in the liquid melt rise to the surface or are carried to the surface by convection and are released into the crystal growth gas ambient and thus have no detrimental effect on the growing ingot. As mentioned above, however, a smaller number of the gas bubbles remain in the liquid melt on or at the crucible bottom or sidewalls throughout the pulling process and when released are grown into the crystal itself during growth. These bubbles, comprised of argon backfilled or argon purge gas, become trapped at the liquid-solid growth interface and cause large crystal voids on the crystal surface. Such defects are generally characterized and detected on sliced silicon wafers as large pits generally having a diameter of greater than about 50 or 100 micrometers. These pits are identified through laser scanning of polished wafers cut from the grown crystal. Such defects can effect 4% or more of wafers sliced from grown crystals and cause these slices to be unfit for grade one wafer product.
As such, a need exists in the semiconductor industry for a process of preparing a silicon melt for growing a single silicon crystal wherein the silicon melt contains a very low amount of gases insoluble in silicon such that a resulting silicon crystal can be grown substantially free of large pits.
Among the objects of the present invention, therefore, are the provision of a process for preparing a silicon melt containing a very low level of gases insoluble in silicon; the provision of a process for preparing a single silicon crystal containing a very low level of large crystal voids; the provision of a process for producing a silicon melt which produces a high percentage of grade one wafers; the provision of a simple, cost-effective process which reduces the number of defects in a grown single silicon crystal; and the provision of a process for preparing a silicon melt in which substantially all of the gas trapped in the silicon charge during the melting process is soluble in silicon.
The present invention, therefore, is directed to a process for controlling the amount of insoluble gas trapped by a silicon melt. The process comprises charging a crucible in a crystal pulling apparatus with polycrystalline silicon and sealing and evacuating the apparatus. Finally, the apparatus is backfilled with a gas selected from the group consisting of nitrogen and hydrogen.
The present invention is further directed to a process for controlling the amount of insoluble gas trapped by a silicon melt. The process comprises charging a crucible in a crystal pulling apparatus with polycrystalline silicon and sealing and evacuating the apparatus. The apparatus is then backfilled with a gas selected from the group consisting of nitrogen and hydrogen and then evacuated again. Finally, the apparatus is again backfilled with a gas selected from the group consisting of nitrogen and hydrogen.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.